Miami County is part of the Dayton Metropolitan Statistical Area.
2007 101,038 2000 98,868 1990 93,182 1980 90,381 1970 84,342 1960 72,901 1950 61,309 1940 52,632 1930 51,301 1920 48,428 1910 45,047 1900 43,105 1890 39,754 1880 36,158 1870 32,740 1860 29,959 1850 24,999 1840 19,688 1830 12,807 1820 8,851 1810 3,841
As of the census of 2000, there were 98,868 people, 38,437 households, and 27,943 families residing in the county. The population density was 243 people per square mile (94/km²). There were 40,554 housing units at an average density of 100 per square mile (38/km²). The racial makeup of the county was 95.78% White, 1.95% Black or African American, 0.19% Native American, 0.79% Asian, 0.01% Pacific Islander, 0.28% from other races, and 1.00% from two or more races. 0.73% of the population were Hispanic or Latino of any race.
There were 38,437 households out of which 33.30% had children under the age of 18 living with them, 59.50% were married couples living together, 9.70% had a female householder with no husband present, and 27.30% were non-families. 23.20% of all households were made up of individuals and 9.50% had someone living alone who was 65 years of age or older. The average household size was 2.54 and the average family size was 2.99.
In the county, the population was spread out with 25.90% under the age of 18, 7.60% from 18 to 24, 28.40% from 25 to 44, 24.80% from 45 to 64, and 13.20% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 96.20 males. For every 100 females age 18 and over, there were 92.40 males.
The median income for a household in the county was $44,109, and the median income for a family was $51,169. Males had a median income of $37,357 versus $25,493 for females. The per capita income for the county was $21,669. About 5.10% of families and 6.70% of the population were below the poverty line, including 9.10% of those under age 18 and 5.60% of those age 65 or over.